1. Technical Field
The invention relates generally to drag bits made from solid infiltrated matrix material impregnated with abrasive particles. More particularly, the invention relates to impregnated bits adapted to drill a hole larger than the diameter of an opening through which the bit can freely pass.
2. Background Art
Rotary drill bits with no moving elements on them are typically referred to as xe2x80x9cdragxe2x80x9d bits. Drag bits are often used to drill very hard or abrasive formations, or where high bit rotation speeds are required.
Drag bits are typically made from a solid body of matrix material formed by a powder metallurgy process. The process of manufacturing such bits is known in the art. During manufacture, the bits are fitted with different types of cutting elements that are designed to penetrate the formation during drilling operations. One example of such a bit includes a plurality of polycrystalline diamond compact (xe2x80x9cPDCxe2x80x9d) cutting elements arranged on the bit body to drill a hole. Another example of such bits uses much smaller cutting elements. The small cutting elements may include natural or synthetic diamonds that are embedded in the surface of the matrix body of the drill bit. Bits with surface set diamond cutting elements are especially well suited for hard formations which would quickly wear down or break off PDC cutters.
However, surface set cutting elements also present a disadvantage because, once the cutting elements are worn or sheared from the matrix, the bit has to be replaced because of decreased performance, including decreased rate of penetration (xe2x80x9cROPxe2x80x9d).
An improvement over surface set cutting elements is provided by diamond impregnated drill bits. Diamond impregnated bits are also typically manufactured through a powder metallurgy process. During the powder metallurgy process, abrasive particles are arranged within a mold to infiltrate the base matrix material. Upon cooling, the bit body includes the matrix material and the abrasive particles suspended both near and on the surface of the drill bit. The abrasive particles typically include small particles of natural or synthetic diamond. Synthetic diamond used in diamond impregnated drill bits is typically in the form of single crystals. However, thermally stable polycrystalline diamond (xe2x80x9cTSPxe2x80x9d) particles may also be used.
Diamond impregnated drill bits are particularly well suited for drilling very hard and abrasive formations. The presence of abrasive particles both at and below the surface of the matrix body material ensures that the bit will substantially maintain its ability to drill a hole even after the surface particles are worn down, unlike bits with surface set cutting elements.
In many drilling environments, it can become difficult to remove the drill bit from the wellbore after a particular portion of the wellbore is drilled. Such environments include, among others, drilling through earth formations which swell or move, and wellbores drilled along tortuous trajectories. In many cases when drilling in such environments, the bit can be come stuck when the wellbore operator tries to remove it from the wellbore. One method known in the art to reduce such sticking is to include a reaming tool in the drilling assembly above the drill bit, or to use a reaming tool in a separate reaming operation after the initial drilling by the drill bit. The use of reamers or other devices to ream the wellbore can incur substantial cost if the bottom hole assembly must be tripped in and out of the hole several times to complete the procedure.
Another, more cost effective method to drill wellbores in such environments is to use a special type of bit which has an effective external diameter (called xe2x80x9cpass throughxe2x80x9d diameter, meaning the diameter of an opening through which such a bit will freely pass) which is smaller than the diameter of hole which the bit drills when rotating. For example, a bit sold under model number 753BC by Hycalog, Houston, Tex., is a xe2x80x9cbi-centerxe2x80x9d bit with surface set diamonds. This bit drills a hole having a larger diameter (called the xe2x80x9cdrill diameterxe2x80x9d) than the pass-through diameter of the bit. Another type of bit is shown in U.S. Pat. No. 2,953,354 issued to Williams et al., which discloses an asymmetric bit having surface set cutters. The structure of a bit such as the one described in the Williams ""354 patent is shown in prior art FIGS. 1 and 2. This bit has an asymmetric bit body. A limitation to bits having surface set cutters is that the cutters are subject to xe2x80x9cpopping outxe2x80x9d of the blades into which they are set. Such bits lose drilling effectiveness when the cutting elements pop out of the blades, as previously explained. Another limitation to the foregoing bits is that they are not well protected against wear in the xe2x80x9cgagexe2x80x9d area of the bit. If the gage area is subject to wear, the bit will drill an undersize wellbore, possibly requiring expensive reaming operations to obtain the full expected drill diameter.
Other prior art bits, such as the bit shown in U.S. Pat. No. 4,266,621 issued to Brock, for example, are eccentric because the axis of the bit body is offset from the axis of rotation. Another way to make an eccentric bit is to radially offset the threaded connection used to connect the drill bit to the bottom hole or drilling assembly. Such bits tend to be dynamically unstable, particularly when drilling a wellbore along a particular selected trajectory, such as when directional drilling, precisely because they are eccentric about the axis of rotation of the drill string.
Generally speaking, the prior art bits are deficient in their ability to withstand a high wear environment in the face area and/or gage area. Accordingly, there is a need for a drill bit which can drill a borehole having a diameter larger than its pass through diameter, which is stable during directional drilling operations, and which is well protected against premature wear on the face of the bit. Additionally, there is a need for a drill bit which can drill a borehole larger than its pass through diameter, which is stable during directional drilling and which is well protected against premature wear in the gage area of the bit to maintain drill diameter.
One aspect of the invention is a drill bit including a bit body and a plurality of blades formed in the bit body at least in part from solid infiltrated matrix material. The blades are impregnated with a plurality of abrasive particles. With respect to an axis of rotation of the bit, one side of the bit body is formed to a smaller radius than an opposite side, so that the bit drills a larger diameter hole than a pass through diameter of the bit.
Another aspect of the invention is a drill bit including a bit body, and a plurality of blades formed in the bit body at least in part from solid infiltrated matrix material. The blades have abrasive cutters thereon. The blades are formed so that, with respect to an axis of rotation of the bit, one side of the bit body is formed to a smaller radius than an opposite side of the bit so that the bit drills a larger diameter hole than a pass through diameter of the bit. The bit further includes a gage sleeve attached to the bit body at a connection end of the bit body.
Another aspect of the invention is a drill bit comprising a bit body, and a plurality of blades formed in the bit body at least in part from solid infiltrated matrix material. The blades have abrasive cutters thereon. The blades are formed so that, with respect to an axis of rotation of the bit, one side of the bit body is formed to a smaller radius than an opposite side of the bit so that the bit drills a larger diameter hole than a pass through diameter of the bit. The blades on at least the opposite side comprise an extended axial length where the blades are formed to the respective one of the radii. In one embodiment, the extended axial length is at least 60 percent of a drill diameter of the bit.
Another aspect of the invention is a drill bit including a bit body, and a plurality of blades formed in the bit body at least in part from solid infiltrated matrix material. The blades have abrasive cutters thereon. The blades are formed so that, with respect to an axis of rotation of the bit, one side of the bit body is formed to a smaller radius than an opposite side of the bit so that the bit drills a larger diameter hole than a pass through diameter of the bit. The blades on the opposite side define a contact angle of at least 140 degrees.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.